A primary objective of photo microlithography is to create images of the finest detail possible. High resolution images having extremely narrow line and space widths permit manufacture of complex integrated circuits on chips of small size.
To form a pattern on a semiconductor chip, light is transmitted through a planar mask and projected onto a planar wafer by a projection lens that generates a reduced image of the mask on the wafer. The projected image causes a chemical reaction in photo-resist material coating the wafer, such that a permanent image of the mask may be created on the wafer. Illumination of the mask plane and of the wafer plane must be uniform, avoiding both small, local variations, and broad macroscopic variations across either plane.
To increase the imaging resolution and decrease the minimum line and space widths, it has been found useful to employ a light source having a short wavelength. Illumination sources in the deep ultra-violet (DUV) range of 150 to 300 nanometers have proven suitable. Because conventional broad-band Xenon and Mercury arc lamps do not generate sufficient energy in the-desired DUV range, other light sources are required.
Krypton-Fluoride (KrF) and Argon-Fluoride (ArF) gas lasers provide efficient illumination in the DUV range and are used for photo microlithography. However, such gas lasers have significant disadvantages. The poisonous nature of the required gases necessitates strict and costly safety measures, and presents a potential safety hazard. Also, gas lasers require regular maintenance during which chip manufacturing must be uneconomically suspended. Required maintenance activities include gas replacement and containment chamber maintenance due to the corrosive effects of the laser gases.
Solid-state lasers avoid the maintenance and safety concerns inherent in gas lasers, while providing sufficient illumination in the desired DUV range. However, a solid-state laser emits a beam that is spatially coherent across its entire diameter. While coherence is advantageous in other applications, it causes problems in photo microlithography. When a coherent beam is expanded or otherwise optically processed to provide even illumination of the mask, speckles or interference fringes will occur in the mask plane. Such non-uniformities are unacceptable because they generate fringes projected onto the wafer. These fringes create corresponding unevenly exposed regions on the wafer.
While an excimer is coherent only locally across small fractional diameter portions of its beam, it is incoherent on the macroscopic scale across its entire beam. The local "islands" of coherency are small, and incoherent with respect to each other. Therefore, a simple "fly's eye" array of short focal-length lenses may be placed in a gas laser beam path to disperse and average the islands of coherency to provide uniform illumination without objectionable speckles or fringes. Such techniques are not effective when used with the entirely coherent output of solid-state lasers.
From the foregoing, it will be recognized that there is a need for an illumination system and method that overcomes these drawbacks by providing (1) uniform illumination in the deep ultra-violet range without speckles or fringes, (2) a reliable system that does not require frequent maintenance or service, and (3) a system that does not require use of hazardous materials.
The present invention satisfies this need by providing an illumination system in which a spatially coherent laser beam is transmitted through an array of frequency-shifting cells. Each cell transmits a different segment of the beam, and shifts the frequency of the respective segment by a particular amount that differs from the frequency shift amount generated by any of the other cells. Each frequency-shifted beam segment is transmitted through a short focal-length lens to diverge the beam segment onto a large area overlapping and generally coextensive with the other similarly diverged beam segments.
The fringes generated by overlapping beams sweep across the large area during the course of one laser pulse. This produces a cumulatively uniform illumination, even though at any instant a particular fringe pattern remains. Because the source beam has a small but significant bandwidth, each beam segment is shifted in frequency by a sufficient amount that differs from the frequency shifts of other beam segments to prevent appreciable overlap of the beam segments within the frequency domain. While each beam segment may generate a speckle pattern at the wafer plane, each segment's pattern is unique. Consequently, the overlapping patterns combine without further interference to provide an averaged, uniform illumination.
To reduce the magnitude of frequency shift required, a first group of the beam segments may be polarized in a first direction, and a second group of segments polarized in an orthogonal direction. Consequently, each beam segment need only be frequency-shifted by an amount necessary to avoid frequency domain overlap with those beam segments of the same polarity, and may tolerate frequency domain overlap with beam segments of orthogonal polarity.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.